1. Field of the Invention
The present invention relates to an actuator driving method, a driving apparatus, and an image forming apparatus.
2. Related Background Art
There are many apparatuses which must drive a plurality of driving objects while synchronizing their driving speeds and the like.
For example, in an electrophotography type image forming apparatus, a motor for driving a photosensitive drum as an image carrier and a motor for conveying a copying paper sheet as a transfer member to the transfer position of the photosensitive drum must be driven in synchronism with each other anytime, e.g., during image transfer, at the start of driving, and at the end of driving.
This is because the photosensitive drum and the transfer member are in contact with or very close to each other, and if their speeds are different, the surface of the photosensitive drum is damaged by friction.
In view of the foregoing, actuators, as drives respectively for driving the photosensitive drum and conveying the transfer member, are controlled in synchronism with each other.
Recently, to improve the image quality of the image forming apparatus, several proposals have been made to increase the rotational precisions of the photosensitive drum and transfer member.
For example, in Japanese Patent Application Laid-open No. 8-95327, the image quality is improved by using a known vibration type motor to drive the photosensitive drum. The vibration type motor transfers vibration energy to a member to be rotated by friction and obtains a driving force. The vibration type motor suffers little play of the shaft due to its structure and generates a large torque during low-speed rotation. This proposal exploits these features to directly connect the photosensitive drum to the shaft of the vibration type motor to drive the drum, thereby reducing rotational variations of the drum and improving the image quality.
A vibration type motor called a resonance type motor generates energy by exciting a vibrator as one building component of the motor around its natural frequency.
The output torque or driving speed of the motor changes depending on the frequency (the number of pulses or the like) of an alternating signal supplied to a piezoelectric element as an electro-mechanical energy conversion element constructing the vibrator.
FIG. 7 shows changes in driving speed of the vibration type motor upon changing the frequency of the driving AC voltage of the vibration motor. In FIG. 7, fr represents the resonance frequency of the vibrator as a building element of the vibration type motor. The vibration type motor exhibits highest speed upon application of an AC voltage having the resonance frequency. The speed vs. frequency curve has different gradients in frequency regions above and below the resonance frequency. In terms of controllability, the vibration type motor is generally better controlled in the frequency region higher than the resonance frequency.
Compared to a conventional electromagnetic motor, the vibration type motor suffers large characteristic variations. For example, as shown in FIGS. 8A and 8B, the vibration type motor has an individual difference in frequency vs. speed characteristics. In FIG. 8A, motors A and B have different resonance frequencies and different speeds even upon application of the same frequency. In FIG. 8B, motors A and C have the same resonance frequency but different gradients of speed vs. frequency curves upon application of the resonance frequency. Owing to these characteristic differences, the speed varies between members connected to a plurality of motors upon application of a specific frequency when the vibration type motor is activated.
Particularly when the vibration type motor is applied to the above-mentioned image forming apparatus, the surface of the photosensitive member may be damaged by friction.
To detect any individual difference of the vibration type motor and operate the motor in accordance with its individual difference, the frequency for driving is swept in advance, characteristics such as the relationship between the frequency for driving and the speed are detected, and then the motor is driven, as disclosed in Japanese Patent Application Laid-open Nos. 59-156168, 63-209481, and 4-251581.
However, when vibration type actuators are used as drivers for the photosensitive drum and the transfer member convey unit, and each motor's characteristics are detected, since the actuators are not synchronized during characteristic detection, a speed difference is produced at the contact portion between the photosensitive drum and the transfer member to damage the surface of the photosensitive drum.